1. Field of the Invention
The present invention relates to panel assemblies in which a core material is sandwiched between layers of composite material and wherein a pigmented top coat is located on the outside of the panel to provide color and a surface finish to the panel. More particularly, the invention is directed to panel assemblies in which the pigmented topcoat is bonded to the panel without the use of a separate adhesive layer or the need for multiple curing steps.
2. Description of Related Art
Honeycomb sandwich panels have been used for many years as interior structural components of aircraft, trains and boats. Due to the need for low levels of fire, smoke and toxic emissions in the event of a fire situation, phenolic resin matrices embedded in a reinforcement fiber are typically used as the panel skin material. Examples of this approach are described in U.S. Pat. No. 4,880,681, U.S. Pat. No. 5,714,419 and WO 92/17331.
In order to provide aesthetic appeal to these panels it is customary to apply a top layer of decorative material such as Tedlar® polyvinyl fluoride film to the outer surface of the panel. These decorative sheets may be patterned as in an aircraft cabin or plain colored as in aircraft luggage holds, galleys and other enclosures and compartments. Current methods of applying the top layer to the panel have a limited process temperature interval due to the nature of the adhesive used to bond the decorative material to the panel's outer surface. This can restrict processing to temperatures of 90 to 130° C. depending upon the adhesive used. Since the resin matrix of the panel is typically cured at a temperature from 140° C. to 160° C., two separate curing steps are required. Two steps are also required to avoid the potential loss of Tedlar texture and the phenolic prepreg “staining” the decorative layer during the cure cycle. This is regarded as a disadvantage as it increases processing times and costs.
Attempts have been made to address this problem. For example, U.S. Pat. No. 4,599,212 discloses a reinforced phenolic laminate panel having a finishing coat of a butylated phenol-formaldehyde resin in which the finishing coat can be mass-pigmented to the desired color. However, in this context mass-pigmented means homogeneously pigmented throughout the whole body of the laminate panel.
JP 2000071365 discloses a method of producing a decorative sheet by successively providing a base material layer, an expanded thermoplastic resin layer containing blowing agents and a colored thermoplastic resin layer containing titanium dioxide such that the top layer shields the yellowing of the expanded foam layer caused by the blowing agents. However, JP 2000071365 does not address fiber reinforced thermoset resin layers.
Pigmented gel coats are known in the composites molding industry, but these are pre-applied to the mold surface either by painting or spraying. Furthermore, gel coats are mixtures of resin and curing agents that are of a different formulation from, but capable of being co-cured with, the resin matrix. Scrim-based gel coats are also known, but these also need to be pre-applied to the mold and contain a high level of resin loading. A disadvantage with gel coats is that they have to be cured in the mold as the first stage of the lay-up and curing process.
Attempts have been made to incorporate titanium dioxide directly into a fiber reinforced phenolic resin matrix, but even with loadings as high as 50%, the desired color intensity was not achieved. In addition, structural integrity such as prepreg to core adhesion deteriorated.